Before pulp can be made into paper, it must undergo several steps referred to as stock preparation, sometimes including slurrying of dried pulp or wastepaper, the blending of different pulps, dilution, refining, beating and the addition of chemicals. The raw stocks used are the various types of chemical pulp, mechanical pulp, and recovered paper and their mixtures. Raw stock is available in the form of bales, loose material or broke, or, in the case of integrated mills, as suspensions. Among virgin pulps, unbleached kraft and hydrophilic fibers such as naturally occurring hemp and solvent regenerated fibers are notably difficult to defiber and disperse. An example of a solvent regenerated fiber is lyocell, in particular 8 mm lyocell fiber. Once dispersed, these hydrophilic fibers have a strong tendency to entangle and knit back together or otherwise form into fiber bundles, sometimes referred to as “nits”.
Stock preparation systems differ considerably depending on the raw stock used and on the quality of the finished stock (furnish) required. For instance, in the case of pulp being pumped directly from a pulp mill, slushing and deflaking stages are omitted. When using dried pulp or wastepaper, on the other hand, pulpers are used to initially disperse the papermaking fibers into a slurry. Pulpers may be operated at low consistency (<5%), medium consistency (6%-8%) or high consistency (10%-30%). The quality of the furnish greatly influences the properties of the paper produced.
In order to produce high quality paper products, it is preferred to avoid papermaking fiber flocs and papermaking fiber nits in the papermaking stock. Flocculation and nit formation in the papermaking stock has been attributed to various factors including relatively long fibers, relatively low fiber coarseness, curled fibers, wide fiber length distribution, fibrillated fiber surfaces, stiff fibers, low viscosity, low shear and fiber charge close to zero. Ideally, it is preferred that individual fibers in the papermaking stock are well dispersed. Papermaking flocs or nits can form or persist during stock preparation or storage prior to being formed into a sheet, even though the fibers are subject to mechanical agitation.
Polyoxyethylene diamine derivatives of diglycidyl ethers are known to be useful in connection with epoxy resins. See U.S. Pat. No. 5,091,574 to Lin et al.; U.S. Pat. No. 7,423,112 to Adkins; U.S. Pat. No. 7,303,992 to Paneccasio et al.; U.S. Pat. No. 5,039,787 to Tanaka et al.; U.S. Pat. No. 4,567,302 to Sivaramakrishnan; and U.S. Pat. No. 4,281,199 to Langdon. Epoxy/polyethylene glycol ether amine resins are reported to be useful as retention aids in U.S. Pat. No. 4,267,059 to Behn et al. Quaternized polyether/oxirane materials have been reported useful as strength aids for paper manufacture. See U.S. Pat. No. 4,198,269 to Evani et al., as well as U.S. Pat. No. 4,156,775, also to Evani et al.
While polymeric additives are well-known in the papermaking art, they have been used only sparingly as aids for dispersing pulps. Achieving a sufficiently uniform sheet has always been a key challenge because cellulose fibers have a high length to thickness ratio and tend to become entangled, forming fiber flocs. See Rojas et al., the Dispersion Science of Papermaking, Journal of Dispersion Science and Technology, Vol. 25, No. 6, pp. 713-732 (2004). Gum-like polymers (i.e., carboxymethylcellulose) can improve dispersion by (presumably) reducing inter-fiber friction; however, such polymers can dramatically reduce drainage and are preferably avoided as a dispersing aid in papermaking operations. The conventional method of choice is thus to apply high hydrodynamic shear to the pulp which is sometimes ineffective, especially with hard to disperse papermaking fibers.
Given the difficulty in repulping recovered paper and dried hydrophilic pulps of fibers such as hemp, flax, cotton and solvent regenerated fibers and the like, there is a need in the art for improved chemical dispersants for dispersing the fibers in papermaking stock. Treating the fibers during stock preparation to avoid nits and flocs is a particular advantage of the present invention. It is also desirable to disperse fiber pulp without unduly increasing the viscosity of the resulting fiber slurry.